The present invention relates in general to digital private branch exchanges (PBX's) and digital transmission facilities which interconnect to digital PBX's and, more particularly, to interface arrangements between digital PBX's and interconnected digital transmission facilities.
Originally, interface arrangements between digital PBX's and digital transmission facilities were provided by the inter-exchange carrier (IXC), for example AT&T, or local exchange company (LEC), for example Ohio Bell. Such interface arrangements can now be provided by customer premise equipment (CPE) vendors and are commonly provided external to the PBX's by functional units referred to as channel service units (CSU's).
On the transmission facility side, a CSU provides a line interface to a DS1 (digital signal level 1) transmission line using T1 signalling wherein a digital signal is transmitted at a nominal rate of 1.544 Mbits/s. On the PBX side, a CSU provides a DSX-1 (digital cross-connect signal) communications port.
The CSU also provides framing circuitry such that digital information can be provided in proper framing format for both the transmission facility and the PBX. The framing circuitry provides transcoding of framing from a Superframe or D4 format to an Extended Superframe (ESF) format; from D4 to D4; from ESF to D4; and, from ESF to ESF such that any variety of PBX is able to communicate with any variety of digital transmission facility.
The CSU also is used to perform maintenance operations for the transmission facility and the PBX. For example, loopback functions and test signals are provided from the CSU. Further, status lamps or light emitting diodes (LED's) and service jacks for gaining access to signals to and from the transmission facility and the PBX are also provided on the CSU.
Unfortunately, normally maintenance procedures require manual intervention at the CSU which requires dispatching a technician to the PBX with attendant costs. While the CSU can be equipped with a network management port which can be remotely accessed for maintenance purposes, such remote maintenance arrangements are expensive, require ESF data link handling which is not integrated with the PBX functions and, accordingly, are not widely used.
There is, thus, a need for an improved port system for interfacing digital PBX's to digital transmission facilities. Preferably, the improved port system would be backward compatible with existing digital PBX switches, could be used to replace existing port systems at reduced cost and would interface digital PBX's to any transmission facility having a DSX-1 communications port and/or to DS1 transmission lines providing all functionality of CSU's. In addition, the improved port system would provide maintenance features which can be remotely accessed as an extension of the PBX into which it is integrated.